My present invention relates in general to antennas having a plurality of independent sources of electromagnetic radiation which are used simultaneously on different wavelengths and in which there is a danger that the circuits associated with several of these sources, situated in the electromagnetic field of one such source, may distort the radiation characteristics of the latter.
More particularly, my invention relates to radar antennas which have a single reflector illuminated by two radiation sources or feeds A and B which operate simultaneously at different wavelengths .lambda..sub.A and .lambda..sub.B, respectively, feed A interfering with feed B. A typical example of such antennas operating on two frequencies can be found in the combination of a surveillance system (secondary radar) with a watching or tracking radar (primary radar).
The function of the primary radar is to detect the presence of a passive object and to supply a number of items of information (range, position, speed) pertaining thereto.
The function of the secondary radar is to transmit interrogation signals which enable the object to be identified when it is fitted with a suitable transponder.
In general, the operating wavelength .lambda..sub.A of the primary radar is in the centimeter range and the wavelength .lambda..sub.B of the secondary radar in the decimeter range.
In normal use, the small value of wavelength .lambda..sub.A and also the desire to optimize the performance of the primary radar make it necessary that priority be given to producing the radiation feed for the primary radar. The phase center of this feed coincides with the focus of the reflector and is thus situated within the conductive envelope defining the space within which are propagated the waves of wavelength .lambda..sub.A used to illuminate the reflector. In the majority of cases, this metal envelope is that of the supply circuits for the feed which are formed by a waveguide terminating in a horn pointing toward the reflector. Thanks to the sheltered location of the phase center, the presence of the elements which form the feed for the radiation of wavelength .lambda..sub.B cause very little interference with the radiation of wavelength .lambda..sub.A.
On the other hand, the radiation from the secondary radar, in its "interrogation" mode, may be considerably distorted by the presence of the external conductive surface of the waveguide structure associated with the feed of the primary radar.
In the "interrogation" mode, the far-field radiation diagram exhibits a maximum (the main lobe) flanked by side lobes of lower levels. Directivity is best in the horizontal or azimuth plane P.sub.H. One and the same elevational plane P.sub.E contains the axes of maximum radiation of both the primary and secondary radars. Plane P.sub.E forms a vertical plane of symmetry for the two sorts of radiation.
The phase center of the feed illuminating the secondary radar must be situated in plane P.sub.E and very close to the focus of the reflector. This requirement is almost always met by dividing the feed into two similar individual sections or groups of individual radiators which are supplied in phase and which are symmetrically positioned about plane P.sub.E. In the majority of cases, however, the external conductive envelope of the structure supplying energy at wavelength .lambda..sub.A to the feed of the primary radar (e.g. a waveguide terminating in a horn) has an axis situated in plane P.sub.E and the two feed sections are thus situated on either side of that envelope.
Being electromagnetically coupled to the waveguide structure, these sections induce in-phase currents in it which radiate in their turn. The two sections, on the one hand, and the waveguide envelope, on the other hand, form a combination of two active antennas energized from a supply and one passive antenna which is not energized.
On account of its structure, the radiation diagram of the waveguide envelope is very different from those of the feed sections. The result is more illumination than intended at the edges of the reflector and an abnormally uniform distribution of energy, with the following consequences:
in the far field, a high side-lobe level; PA1 a not inconsiderable level of energy radiated by the waveguide envelope outside the reflector, and hence the appearance of high-level side lobes in directions well off the main axis of radiation; PA1 a main lobe which is deformed by reason of faulty focusing due to the displacement of the phase center; PA1 a loss of gain; PA1 a worsening of the transverse polarization factor.
It has been possible hitherto to correct these faults to some degree, during construction, by laborious mechanical and electrical adjustments arrived at by a process of trial and error, as by shifting the two feed sections, modifying their phase difference, or altering the position of the envelope.
This, however, is no more than a palliative since the method amounts to shaping the radiation diagram of the secondary radar to a more or less satisfactory form by means of a series of compensations which are valid for one structure and a given set of dimensions but which cannot form a basis for a method of correction applicable to other structures or dimensions, even if they are quite closely related.